Cycle of reproduction, oocyte atresia, and maturation size of the Manila clam Ruditapes philippinarum on a pebble beach in Mutsu Bay, Northern Japan

ABSTRACT This study investigated the seasonal cycle of reproduction and oocyte atresia (i.e. cell death and degeneration), and size at maturity of the Manila clam Ruditapes philippinarum (Mollusca: Bivalvia) in Mutsu Bay, Northern Japan. Monthly changes in condition factor and reproductive stage of gonads indicated that females and males spawned during July–September and June–October in 2014, respectively. The volume fraction of atretic oocytes reached a small peak at 9.3% in August (i.e. cell death of oocytes during their development in the spawning period) and the largest peak in October at 25.0% (i.e. residual oocytes in the recovery period). The atretic oocytes under resorption by macrophage-like cells comprised 2.5–14.3% and 100% of total atretic oocytes in the spawning and recovery/early gametogenesis periods, respectively. The shell length at 50% maturity was estimated to be 21.1 mm in females and 18.5 mm in males.